This invention relates to fabrication of high-density interconnect (HDI) electric or electronic modules, and more particularly to arrangements for encapsulating the components of modules in which at least one of the components requires connections to the xe2x80x9cbackxe2x80x9d side opposite to that side of the component bearing an array of electrical connection pads.
High density interconnect (HDI) modules are modules which take advantage of the high circuit densities of semiconductor or solid-state chips (dies) or integrated circuits (or other components), by allowing many chips to be mounted close together on a planar surface, with a multilayer interconnect structure overlying the connection sides of the chips to provide the electrical interconnections which allow interaction of the chips to generate complex systems.
In the past, HDI modules have been fabricated by defining a plurality of wells in a ceramic substrate, and mounting the chips in the wells with the planes of their connection pad arrays aligned. A dielectric film or sheet is laminated over the connection pads of the chips, following which through vias are laser-drilled to those connection pads to which connection is to be made. A pattern of electrically conductive metal is deposited over a portion of the exposed dielectric film and into the vias to make connection to the underlying chip contacts. Additional layers of interconnect are made in much the same manner, thereby providing electrical conducting paths between and among the chips sufficient for assemblages of any complexity, and suitable for high-frequency operation. Where an electrical connection to the backside of a die is desired, this has been provided by patterning metal traces on the ceramic substrate, and bonding the die to the underlying metal with a conductive adhesive.
A more recent method for fabricating such modules is the xe2x80x9cchip-on-flexxe2x80x9d method, in which a dielectric film is mounted to a supporting frame under tension. The film may be patterned on either or both sides with layers of electrically conductive circuit paths of the interconnect circuitry. The dielectric film is then coated with an adhesive layer, and used as the bonding surface onto which the chips of the array of modules are attached, with their connection pads facing the dielectric film. After bonding the die to the dielectric film, a region containing the array of modules is defined by a metal collar, and the region within the collar is filled with granular encapsulant material. The granular encapsulant material is cured by heating, which melts and cures the encapsulant. Additional interconnection layers can then be applied to thereby complete the desired individual modules. The completed modules are then physically separated from each other by a dicing process, such as a wafer saw or by use of a laser. While it is possible to separate the modules from each other at any time after the encapsulant material is cured, it is preferable to add any additional interconnection layers prior to any dicing step.
While the chip-on-flex method provides several advantages over the prior HDI process, the encapsulation method has several shortcomings. The encapsulation process forms a uniform thickness of encapsulant material across all the modules due to its liquid state prior to cure. This uniform thickness fully encapsulates all embedded components. Electrical connection(s) to the backside of a die through the encapsulant are not possible. Previous efforts to address this issue have involved partial removal of cured encapsulant material to expose the backsides of the components, as for example by milling or ablating the cured material exclusively in the region of selected chips. This procedure is difficult, since the exact positions of the chips are typically not visible during the milling or ablating. If the entire module is lapped or ground, the backsides of the components will be subjected to shearing forces which may damage the components. In addition, all components of the thickness to which the module is ground will be exposed, including components for which full encapsulation is desired.
Another shortcoming of the chip-on-flex method is that the encapsulation material, when applied to encapsulate an area which is several times larger than that of a discrete module, has the undesired tendency to distort the underlying dielectric film andor to stress the chips. The distortion makes further processing difficult, and the stresses tend to reduce the reliability of the resulting module.
Improved chip-on-flex encapsulating methods are desired.
A method according to an aspect of the invention for molding multichip modules in such a manner as to allow backside contact includes the step of procuring a tensioned dielectric film or sheet populated with components having electrical contacts adjacent the film and also having at least one of a backside thermal contact and a backside electrical contact. One or more module region(s) is/are defined by dispensing a high-viscosity liquid dam material around those components included within a single module. The dam defines exterior dimensions substantially equal to the finished size of the module, unless trimming steps are added. At least a portion of the dammed region is filled with liquid encapsulant material by use of an programmed automatic dispenser apparatus which relatively moves a dispenser tip or nozzle along a preprogrammed path within the module region. In one version of the invention, the automatic dispenser apparatus is controlled to define a dispenser or dispensing path which in plan view (a) begins dispensing near the geometric center of the module region, and continues dispensing in a generally spiral overall pattern, so that, in the absence of components requiring backside contacts, the entirety of the dammed region is covered or filled with encapsulant, (b) stops the dispensing in those regions occupied by those of the components for which backside contact is desired. In one mode of the method, the dispenser tip is maintained at a fixed height during dispensing. In another mode of the invention, the controlling step includes the further step of (c) raising the dispenser tip and stopping the dispensing at any components which are not to be encapsulated, or which are to have their back sides clear of encapsulant material. In a further version of the method, the step of defining a module region includes the step of dispensing the encapsulant in high-viscosity form. In another variant, additional dammed regions may be introduced within the bounds of a given module, to thereby define unencapsulated regions. These unencapsulated regions may be populated or unpopulated.